Following the Pacer withdrawals, there are now plenty of 2-car 156s and 158s operating on routes where a 2-car 195 would provide adequate seating capacity. Examples: Cumbrian Coast, Carlisle to Newcastle, Settle & Carlisle.
If the 195/0s were swapped on to these routes, the 15x could be redeployed in 3- and 4-car formations, with inter-unit gangways, instead of using paired-up 195/0s on routes that need the extra capacity.
If there are too many 150/1s to operate them all on 2-car diagrams, some could be reformed into 3-car units, as per the Arriva franchise agreement, with some of the 150/2s split up to provide the centre cars.
IMO the aim should be to minimise the operation of the DMUs lacking end gangways (150/1s and 195s) in multiple formations.
The 150's should be reformed into a mixture of three and four car units and stuck on to services in the North West and used on a small number of services in Yorkshire. Whatever's left gets broken down into spare parts units. Units would remain as this before they are either scrapped or preserved (hopefully all the 150/1's and /2's will be scrapped leaving the two prototypes being retained for preservation).
The 153's, retain seven units and insert into the middle of the seven class 155's (341 - 347) to make then into three car units and reform the rest into three car class 155/9's - the only difference with a /9 variation would be the one coach being PRM'd, four cabs being removed, and the 'new cab' being used at either end to visually display that this is a /9 variation of the class.
The 156's, again reform into three car units. As there's be a number of units that would end up with either just a single bog or with two, two subclasses would have to be used - /8's for the units with two bogs and /9 for the units with just one.
The 158's, any two car unit gets reformed into a three car hybrid.
With more reformed units, you've got a bit more capacity space - especially if/when Fleetwood, Wetherby, Ashington and a host of other lines in the north are reopened as it gives them the fleet required for the services. Leaving the 170's and 195's running the more established services until the 150's are scrapped and replaced with presumingly more 195's.
More centre coaches should be ordered for the 195's to make them into a mixture of four and five car units.